The NAVSTAR global positioning system (GPS) is a collection of 24 earth-orbiting satellites. Each of these GPS satellites travels in a precise orbit about 11,000 miles above the earth's surface. A GPS receiver locks onto at least 3 of the satellites, and responsive, thereto, is able to determine its precise location. Each satellite transmits a signal modulated with a unique pseudo-noise (PN) code. Each PN code comprises a sequence of 1023 chips that are repeated every millisecond consistent with a chip rate of 1.023 MHz. Each satellite transmits at the same frequency. For civil applications, the frequency is known as L1 and is 1575.42 MHz. The GPS receiver receives a signal that is a mixture of the transmissions of the satellites that are visible to the receiver. The receiver detects the transmission of a particular satellite by correlating the received signal with shifted versions of the PN code for that satellite. If the level of correlation is sufficiently high so that there is a peak in the level of correlation achieved for a particular shift and PN code, the receiver detects the transmission of the satellite corresponding to the particular PN code. The receiver then used the shifted PN code to achieve synchronization with subsequent transmissions from the satellite.
The receiver determines its distance from the satellite by determining the code phase of the transmission from the satellite. The code phase (CP) is the delay, in terms of chips or fractions of chips, which a satellite transmission experiences as it travels the approximately 11,000 mile distance from the satellite to the receiver. The receiver determines the code phase for a particular satellite by correlating shifted versions of the satellite's PN code with the received signal after correction for Doppler shift. The code phase for the satellite is determined to be the shift that maximizes the degree of correlation with the received signal.
The receiver converts the code phase for a satellite to a time delay. It determines the distance to the satellite by multiplying the time delay by the velocity of the transmission from the satellite. The receiver also knows the precise orbits of each of the satellites. The receiver uses this information to define a sphere around the satellite at which the receiver must be located, with the radius of the sphere equal to the distance the receiver has determined from the code phase. The receiver performs this process for at least three satellites. The receiver derives its precise location from the points of intersection between the at least three spheres it has defined.
The Doppler shift (DS) is a frequency shift in the satellite transmission caused by relative movement between the satellite and the receiver along the connection-of-sight (LOS). It can be shown that the frequency shift is equal to             v      LOS        λ    ,where vLOS is the velocity of the relative movement between the satellite and receiver along the LOS, and λ is the wavelength of the transmission. The Doppler shift is positive if the receiver and satellite are moving towards one another along the LOS, and is negative if the receiver and satellite are moving away from one another along the LOS.
The Doppler shift alters the perceived code phase of a satellite transmission from its actual value. Hence, the GPS receiver must correct the satellite transmissions for Doppler shift before it attempts to determine the code phase for the satellite through correlation analysis.
The situation is illustrated in FIG. 1, which shows a GPS receiver 10 and three GPS satellites 12a, 12b, and 12c. Each satellite 12a, 12b, 12c is transmitting to the GPS receiver 10. Satellite 12a is moving towards the GPS receiver 10 along the LOS at a velocity va+14; satellite 12b is moving away from the GPS receiver 10 along the LOS at a velocity vc−16; and satellite 12c is moving away from the GPS receiver 10 along the LOS at a velocity vc−18. Consequently, assuming a carrier wavelength of λ, the transmission from satellite 12a will experience a positive Doppler shift of             v      a      +        λ    ;the transmission from satellite 12b will experience a negative Doppler shift of             v      b      -        λ    ;and the transmission from satellite 12c will experience a negative Doppler shift of             v      c      -        λ    .
The GPS receiver 10 functions by sampling a finite portion of the received signal 20 and then processing the samples. Typically, external constraints limit the size and occurrence of the sampling period. For example, in the case of a mobile wireless phone integrated with a GPS receiver 10, the sampling window should be limited to those periods in which the phone is not transmitting. The purpose is to avoid interference between the transmitter and the GPS receiver 10.
The problem is that the signal-to-noise (S/N) ratio of the received signal 20 over a finite sampling window may not be sufficient to detect the presence and range of the satellites. For example, the signal may be such that there is no correlation value for a particular set of hypotheses which is significantly larger than the correlation values resulting from the other hypotheses tested.
Moreover, it is difficult to combine segments of samples captured over different periods of time because each is subject to a different code phase which must be accounted for before the segments can be combined, and these code phases are unknown. In an effort to increase the SNR of the received signal 20, prior art receivers are thus required to either forgo operation during times in which the received signal 20 is weak, or to extend the sampling period beyond the limits imposed by external constraints. In certain applications, such as the case of a GPS receiver 10 integrated with a mobile wireless phone, extension of the sampling window is not usually feasible since it would subject the received signal to unacceptable interference from the phone's transmitter. In such applications, the practical effect is to forego operation of the GPS receiver 10 when the received signal 20 is weak. Such occurrences are frequent because of the approximately 11,000 mile distance traveled by the GPS satellite transmissions, because of blockage, multipath fading or attenuation due to buildings, terrain or trees, and because of the noise to a particular satellite represented by the other satellite's transmissions.
Consequently, there is a need for a signal detector that overcomes the disadvantages of the prior art. Similarly, there is a need for a GPS receiver 10 that overcomes the disadvantages of the prior art.